Ophthalmic lenses (or spectacle lenses) are supplied to the wearer in a mounting frame, so that the lenses are at a distance from the wearer's eyes. To look at a point in space, the wearer needs to move his or her head, perform an eye movement or combine a movement of the head and eyes. For example, in order to read a book of ordinary size, the wearer's head remains most frequently stationary and only the eyes move to follow the lines of text; as against this, when a spectacle wearer is driving a vehicle, he or she will generally employ a combination of head and eye movements.
For prescribing multifocal progressive lens spectacles, methods for customized prescription have been envisaged for many years now aimed at individually calculating or choosing the progressive lens design which is best adapted to the dynamic behavior of each spectacle wearer, notably in terms of head and eye movement.
An individual test makes it possible to assign to each individual wearer a coefficient that qualifies his propensity to rather move the head or rather move the eyes (head mover/eye mover). This behavioral coefficient presides over calculation of the best suited design. Thus, for example, for a wearer who tends to move the eyes rather than the head, a design having a fairly wide sharp vision field will be chosen. As against this, for a wearer who rather tends to move the head, and the eyes less, gentle progression at the periphery will be chosen to avoid an impression of swaying. Progression length can also be varied.
In practice, a critical step which is somewhat difficult to implement both from a technical as well as from a commercial point of view proves to be the individual test allowing the behavioral trend of each individual wearer to be determined. There has arisen the particular need for a device making it possible to perform in a simple, reliable and repetitive manner, measurements of head and eye movements of the spectacle wearer.
There do indeed exist already on the market devices for performing this measurement. Such devices mainly comprise:                a fixed portion comprising several point light sources adapted to be activated alternately and distributed along a horizontal line, one central one facing the wearer and other side ones at each side of the central source at defined locations, and        a mobile portion located on the wearer's head a bit like a hat and of which the angular position about the vertical is determined with respect to the fixed portion.        
When a source at the side is lit up, the wearer will look at it by turning the head to a greater or lesser degree depending on his behavioral habits. one now measures the angle of rotation of the head and this is representative of the wearer's propensity to rather turn the head or the eyes when his view is drawn laterally.
One example of a device of this type and its implementation is discussed in the article “Head movement propensity” by James H Fuller, Experimental Brain Research 1992, pp 152-164. More recently, there has been a proposal to use, in a similar measurement method, modern wireless sensors of the type sold under the name 3SPACE® FASTRAK® by Polhémus Incorporated, Colchester, Vt., USA and described in the brochure “User's manual revision F”, OPM3609-002 C, November 1993, or any more recent edition thereof.
These existing devices are satisfactory from a technical point of view but in practice are too expensive and complex or too impractical to operate for widespread use by opticians. There does exist a need for a simpler and less expensive measurement device.
More generally, for ophthalmic lens design, it can be useful to characterize the behavior of a wearer or given population. There is consequently a need for systems and methods allowing the behavior of an ophthalmic lens wearer to be characterized or measured.